The changes in NAD ion/NADH ratios during cell growth in normal and transformed cells has been investigated. In the presence of glucose that ratio is 2 to 3, removal of glucose increases the ratio 4-fold, as does the substitution of galactose for glucose. No other hexose or carbon source can substitute for glucose completely for growth. The redox state varies widely dependent on the hexose used as the carbon source. There are striking changes in cell morphology associated with different sugars. Differences in rate of sugar transport could not explain these differences; nor could the activity of hexokinase explain the differences observed between fructose and glucose.